1. Field of the Invention
The present invention relates to a process for producing graphite electrodes, and more particularly to a process for efficiently producing graphite electrodes for thin batteries which are suitable as power sources for various electronic devices of small size or thickness.
2. Description of the Prior Art
In preparing graphite electrodes for thin batteries or the like, iron group elements are usually used as the substrate material for depositing graphite thereon by thermal decomposition since these elements act catalytically in the thermal decomposition of various hydrocarbon compounds and further ac to promote graphitization of carbon (Journal of Catalysis, 29, 15-19(1973), Carbon, 10, 601-611(1972)). However, when the substrate is in the form having a small specific surface area such as a common plate, it is impossible to achieve a high enough deposition rate of graphite for practical use. The rate does not become high enough even if a reticular substrate is used. Accordingly, sponge metals having a large specific surface area are used solely as substrates. For example, a vapor growth process is known wherein graphite is deposited on sponge nickel by thermal decomposition (Unexamined Japanese Patent Publications SHO63-102167 and SHO 63-230512).
Nevertheless, such a usual process for preparing graphite electrodes involves difficulty in controlling the amount of deposition of graphite on the surface of sponge metal because of great variations in the specific surface area thereof. Consequently, the graphite electrodes obtained vary greatly in characteristics and are not satisfactorily usable.
Further, for the efficient growth of graphite on the surface of sponge nickel substrate, the substrate needs to have a considerably large overall surface area and must, therefore, have a thickness at least over 1 mm. In order to obtain thin electrodes, accordingly, the product of the process needs to be pressed eventually to the thickness, and the pressing step breaks or releases the graphite layer formed resulting in the problem of reduced capacity of the graphite electrode.
The present invention, which has been accomplished to overcome the foregoing problems, contemplates provision of a process for producing with ease and efficiently graphite electrodes having a high capacity and below 1 mm in thickness.
From the above viewpoint, we have conducted intensive research and, consequently, prepared a graphite electrode by forming on a substrate a deposition layer of fine globular graphite particles larger than usual graphite particles formed on an electrode substrate, covering the deposition layer with a thin metal film of an iron group element or the like to form a surface having minute projections, and forming a usual graphite layer on the surface by the thermal decomposition CVD process. We have found that this graphite layer can be formed at a remarkably improved deposition rate and have accomplished the present invention.